The first thing I'd say is that Veritaserum is only one tool. It may be magic, but it is not invincible, it is not infallible. Rowling has gone on record saying:
[Veritaserum works best on] the unsuspecting, the vulnerable and those insufficiently skilled (in one way or another) to protect themselves against it.
As such, in no sense can Veritaserum be relied upon to always force the truth out of someone. That, I think, is worth bearing in mind.
Secondly, the magic of the Fidelius Charm to me is rather beautiful. It's about fidelity, loyalty, trust and friendship.
'An immensely complex spell,' he said squeakily, 'involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret Keeper, and is henceforth impossible to find - unless, of course, the Secret Keeper chooses to divulge it. As long as the Secret Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting-room window!'
Harry Potter and the Prisoner of Azkaban - pp.152-3 - Bloomsbury - Chapter 10, The Marauder's Map
To me, the element of choice there is all-important. The spell is broken if your Secret Keeper betrays you. It's about betrayal, infidelity, treachery.
Of course, you could choose to betray someone under duress. I would argue that if the information was tortured out of you - well that would be forgiveable, but you have still elected to give your friends up. I mean, the torture might send you out of your mind, you might lose all control and hardly know what you were saying any more, but it's still a decision you take. Overriding that autonomy with a potion that would just send your secrets pouring out of you would be different, to me. That wouldn't be betrayal. If I'm tortured into giving up your whereabouts, I have given you up - however understandably. But if somebody finds where you are by searching my house and discovering the information - well, I've been a bit careless, but I have not given you up.
So I would argue that to break the Fidelius Charm, it would have to be elective. You would have to make a decision - however constrained and inevitable that decision was. As such, Veritaserum, which would not enable you to make a choice, would be defeasible by that. It would not be able to get that information out of you.
So, Veritaserum can be fought by certain means. If you do not have those means or have failed to employ them, it will reveal your secrets without you having any control over them. As such, when it works, it cannot force a betrayal. As such, it cannot break a Fidelius Charm. As such, that is another means by which it may be fought.
Having said that, I think it's an interesting question whether extreme torture would also be vulnerable to that. I would argue that if the torture got so extreme that you lost all control, that too might not break the Fidelius Charm, because it's not a choice. However, up to a certain point in the torture (and it gives me no pleasure to speculate on where that line might be drawn), in giving up the information, you would be choosing to sacrifice your friends' lives to save you from pain. Which would be betrayal, however understandable. Which would break the Charm.